Alamouti-Space Time Block Coding (STBC) and Alamouti-Space Frequency Block Coding (SFBC) aim to enhance a reception performance, compared to a conventional single antenna system, by virtue of a spatial diversity in a fading channel by coding a signal in time and space axes or frequency and space axes using multiple antennas at a transmitter. For instance, according to the Alamouti-STBC, the transmitter of FIG. 1 encodes a signal channel-encoded at a channel encoder 101 through an Alamouti-STBC encoder 103 in the time and space axis, and then transmits two transmit symbols via two transmit antennas 105 and 107 in symbol periods at the same time. The receiver receives the signal of the two transmit symbols via receive antennas.
As the enhanced reception performance using the Alamouti-STBC leads to the increase of a system capacity and the extension of a cell coverage, the Alamouti-STBC is adopted to various commercial systems such as mobile WiMax system or Long Term Evolution (LTE) system.
Usually when a transmitter of a communication system transmits a signal using the Alamouti-STBC, a receiver can receive the signal and differ the decoding scheme according to the channel condition. The receive signal of the receiver in the system using the Alamouti-STBC is expressed as Equation 1:
                              y          =                      Hx            +            u                          ,                                  ⁢                              [                                                                                                      y                      1                                        ⁡                                          (                      1                      )                                                                                                                                                              y                      1                      *                                        ⁡                                          (                      2                      )                                                                                                                                                              y                      2                                        ⁡                                          (                      1                      )                                                                                                                                                              y                      2                      *                                        ⁡                                          (                      2                      )                                                                                            ]                    =                                                    [                                                                                                                              h                          11                                                ⁡                                                  (                          1                          )                                                                                                                                                              h                          12                                                ⁡                                                  (                          1                          )                                                                                                                                                                                                  h                          12                          *                                                ⁡                                                  (                          2                          )                                                                                                                                    -                                                                              h                            11                            *                                                    ⁡                                                      (                            2                            )                                                                                                                                                                                                                            h                          21                                                ⁡                                                  (                          1                          )                                                                                                                                                              h                          22                                                ⁡                                                  (                          1                          )                                                                                                                                                                                                  h                          22                          *                                                ⁡                                                  (                          2                          )                                                                                                                                    -                                                                              h                            22                            *                                                    ⁡                                                      (                            2                            )                                                                                                                                              ]                            ⁡                              [                                                                                                    x                        1                                                                                                                                                x                        2                                                                                            ]                                      +                                          [                                                                                                                              u                          1                                                ⁡                                                  (                          1                          )                                                                                                                                                                                                  u                          1                          *                                                ⁡                                                  (                          2                          )                                                                                                                                                                                                  u                          2                                                ⁡                                                  (                          1                          )                                                                                                                                                                                                  u                          2                          *                                                ⁡                                                  (                          2                          )                                                                                                                    ]                            .                                                          [                  Eqn          .                                          ⁢          1                ]            
In Equation 1, y denotes a receive signal vector, yi(t) denotes a receive signal in the t-th symbol period (or the t-th subcarrier) over the i-th receive antenna, H denotes a channel matrix, and hij(t) denotes a channel gain in the t-th symbol period (or the t-th subcarrier) between the i-th receive antenna and the j-th transmit antenna. Further, x denotes a transmit symbol vector, xj denotes a transmit symbol over the j-th transmit antenna, u denotes a background noise, and ui(t) denotes a background noise in the t-th symbol period (or the t-th subcarrier) at the i-th receive antenna.
When the channel in the period adopting the Alamouti-STBC is a static channel; that is, the channel is constant in successive symbol periods (or successive subcarriers), the Alamouti-STBC attains orthogonality as expressed in Equation 2. Accordingly, the receiver can distinguish the two transmit symbols, to ensure the optimal reception performance and design the receiver with the low implementation complexity.
                                                        Assumption              ⁢                              :                            ⁢                                                          ⁢                              h                11                                      =                                                            h                  11                                ⁡                                  (                  1                  )                                            =                                                h                  11                                ⁡                                  (                  2                  )                                                              ,                                    h              12                        =                                                            h                  12                                ⁡                                  (                  1                  )                                            =                                                h                  12                                ⁡                                  (                  2                  )                                                              ,                                    h              21                        =                                                            h                  21                                ⁡                                  (                  1                  )                                            =                                                h                  21                                ⁡                                  (                  2                  )                                                              ,                                    h              22                        =                                                            h                  22                                ⁡                                  (                  1                  )                                            =                                                h                  22                                ⁡                                  (                  2                  )                                                                    ⁢                                  ⁢                              H            H                    =                      [                                                                                h                    11                    *                                                                                        h                    12                                                                                        h                    21                    *                                                                                        h                    22                                                                                                                    h                    12                    *                                                                                        -                                          h                      11                                                                                                            h                    22                    *                                                                                        -                                          h                      21                                                                                            ]                          ⁢                                  ⁢                                            x              ^                        =                                          H                H                            ⁢              y                                ,                                    [                                                                                                                  x                        ^                                            1                                                                                                                                                          x                        ^                                            2                                                                                  ]                        =                                                            [                                                                                    α                                                                    0                                                                                                            0                                                                    α                                                                              ]                                ⁡                                  [                                                                                                              x                          1                                                                                                                                                              x                          2                                                                                                      ]                                            =                              [                                                                                                    v                        1                                                                                                                                                v                        2                                                                                            ]                                                    ⁢                                  ⁢                  α          =                                                                                      h                  11                                                            2                        +                                                                            h                  12                                                            2                        +                                                                            h                  21                                                            2                        +                                                                                                  h                    22                                                                    2                            .                                                          [                  Eqn          .                                          ⁢          2                ]            
As given by Equation 2, assuming the static channel indicative of less channel variation in the successive symbol periods (or the successive subcarriers), the receiver decodes the two transmit symbols independently.
However, in the reception assuming the static channel environment, when the channel is varied and the static channel environment is not sustained, interference signals are generated between the two transmit symbols and thus the performance is degraded. For the non-static channel environment, a method for receiving the transmit symbol with the Alamouti-STBC adopted is demanded.
When the channel in the period of the Alamouti-STBC is the non-static channel; that is, when the channel is inconstant over the successive symbol periods (or the successive subcarriers), the Alamouti-STBC does not obtain the orthogonality. The receive signal is similar to the receive signal of a Spatial Multiplexing mode which employs two transmit antennas and four receive antennas. When the transmitter uses the Alamouti-STBC and the channel is inconstant, the conventional receiver adopts the SM reception scheme such as Minimum Mean Squared Error (MMSE).
Typically, the system using the Alamouti-STBC also supports the SM technique using the multi-antenna. Herein, the SM technique raises a data transfer rate by transmitting independent data over the multiple antennas at the same time.
In a system using the same number of transmit antennas and receive antennas, when the transmit symbol of the Alamouti-STBC is decoded according to the SM reception scheme, the magnitude of a channel matrix used is greater than that of the channel matrix of an SM receiver. For example, in the system using two transmit antennas and two receive antennas, the SM receiver uses 2×2 channel matrix. By contrast, 4×2 channel matrix is used to decode the transmit symbol of the Alamouti-STBC according to the SM reception scheme.
Accordingly, as shown in FIG. 2, even though the receiver of the conventional communication system supports the Alamouti-STBC and the SM at the same time, an Alamouti-STBC decoder 201 including the MMSE using the 4×2 channel matrix is designed separately from an SM decoder 203. In this situation, the complexity of the receiver increases.